DIY EcoComplete/Floramax. You did a nice job pulverizing the lava rock. How long did it take you to get that cup full?
You've broken down the size of the lava rock, but it's still lava rock. (vs. volcanic soil) Using root tabs will be just great.
The differences between volcanic soils and lava rock is the elapsed time. Volcanic soil has been weathered & broken down over years and also has organic matter and other compounds (volcanic ash, humus, sands, etc.) in it.